In ionographic devices such as that described by U.S. Pat. Nos. 4,524,371 to Sheridon et. al. or 4,463,363 to Gundlach et. al., an ion producing device generates ions to be directed past a plurality of modulation electrodes to an imaging surface or electroreceptor in imagewise configuration. In one class of ionographic devices, ions are produced at a coronode supported within an ion chamber, and a moving fluid stream entrains and carries ions produced at the coronode out of the chamber. At the chamber exit, a plurality of control electrodes or nibs are modulated with a control voltage to selectively control passage of ions through the chamber exit. Ions directed through the chamber exit are deposited on a charge retentive surface in imagewise configuration to form an electrostatic latent image developable by electrostatographic techniques for subsequent transfer to a final substrate. The arrangement produces a high resolution non-contact printing system. Other ionographic devices are known which operate similarly, but do not rely on a moving fluid stream to carry ions to a surface.
The generation of high quality images requires uniform electroreceptor motion past the writing head to prevent banding due to unwanted fluctuations in optical density or other motion related defects in the image. These defects have been reduced in related printing arts by synchronizing data flow to surface motion. Commonly, an encoder or an equivalent device is used to incrementally measure surface motion and initiate writing when the surface has advanced by one print line width. Since the writing process is linked to surface motion, each line in the image is correctly placed on the surface in spite of velocity variations.
However, in ionographic printing, although each line of the image may be correctly located on the electroreceptor surface in accordance with the above encoding arrangement, so that no banding occurs because of line spacing variations, fluctuations in local charge density occur due to differences in dwell time of the printhead at each line. The dwell time varies because the time between encoder pulses varies with surface velocity. With varying length of the writing period, charge is deposited on the electroreceptor in varying amounts, which ultimately develop as darker and lighter lines, in a non-uniform manner.
U.S. Pat. No. 4,839,671 to Theodoulou et. al. teaches control of print density or resolution (number of dots per inch) by providing a set of control electrodes individually driven by a clocked driver element, wherein the clocking signals are synthesized based on desired resolution. U.S. Pat. No. 4,575,739 to De Schamphelaere et. al. shows a motion-based control system for modulating a set of illuminating discharge devices in an electrophotographic printer in accordance with motion of the photoreceptor, based on the desired optical density level of the image. U.S. Pat. No. 4,494,129 to Gretchev shows an ionographic printer where gated oscillators apply a high voltage alternating current to drive electrodes and a counter provided for controlling the gated oscillator. U.S. Pat. No. 4,435,723 to Seimiya shows a recording apparatus where derived image signals are switched for blocking or allowing passage of image signals when predetermined image coordinates coincide with detected position to provide an editing or erase function. U.S. Pat. No. 4,435,066 to Tarumi et. al. discloses an ion modulating electrode which increases the strength of the ion flow passing therethrough by strengthening the electric field between the electrode and the charge retentive surface. U.S. Pat. No. 4,287,524 to Ohnishi et. al. shows an electrostatic recording head where the control voltage is delayed after application of the recording voltage to produce a potential difference large enough to prevent a decrease in recording density. U.S. Pat. No. 3,958,251 to Borelli teaches an electrode driving circuit in an electrographic head which maintains the electrodes active when the print medium is moved across it to produce a latent image. U.S. Pat. No. 3,815,145 to Tisch discloses an electrostatic printing system in which the ion stream is controlled by an electrical and mechanical shuttering arrangement. U.S. Pat. No. 4,804,980 to Snelling shows an ionographic printer where modulation of the ion beam directed at a charge retentive surface is accomplished with a laser writing beam writing on a cylindrical photoconductive screen, arranged to bring exposed portions thereof between the ion source and charge retentive surface for the creation of latent image thereon. U.S. patent applications Ser. Nos. 07/370,317, filed 6/22/89, U.S. Pat. No. 4,972,212 by Hauser et. al. and 07/428,714, U.S. Pat. No. 4,973,994 filed 10/30/89 by Schneider, both teach electrodes adjacent the ion stream path and between the modulation electrodes and the imaging surface for control of blooming artifacts, and 07/428,714 filed 10/30/89, U.S. Pat. No. 4,973,994 by Schneider, teaches varying bias on such an electrode. All the patents and patent applications cited are incorporated by reference herein.